1. Field of the Invention
The present invention relates to a semiconductor device such as a double-diffused metal-oxide field-effect transistor (MOSFET) and more particularly to a semiconductor device having a structure in which a parasitic transistor formed in the MOSFET is difficult to operate. Furthermore the present invention also relates to a method of manufacturing a semiconductor device having such a structure.
2. Description of the Related Art
In a semiconductor device, such as a double-diffused MOSFET or IGBT (insulated gate bipolar transistor), a parasitic transistor is formed internally. In case where the parasitic transistor operates, functions inherent in the device will be prevented. Depending on operating conditions, the parasitic transistor may lead to breakdown of the device.
The double-diffused MOSFETs are widely used in applications requiring a higher speed of operation than bipolar transistors. For example, the MOSFETs find application in controlling electric motors. The switching of an inductive load, such as an electric motor, may operate a parasitic transistor existing within a MOSFET and thus lead to breakdown of the MOSFET. The reason of the breakdown of the MOSFET is that abrupt variations in voltage and current resulting from switching of the inductive load cause current flow into a base region of the parasitic bipolar transistor, which is thus caused to operate, and excessive current flows locally in the transistor. Therefore, there is a great need for a solution to the problem of to parasitic transistors.